The present invention relates to a hydraulic control method of an automatic transmission and to an apparatus thereof. More specifically, the invention relates to a hydraulic control for discharging air that has been mixed into a hydraulic circuit.
There has been known an automatic transmission having a plurality of friction engagement elements that are engaged by the supply of a hydraulic pressure, wherein a hydraulic control is executed to discharge air that has been mixed into a hydraulic circuit by supplying the hydraulic pressure to the friction engagement elements that are to be released based on a request in a gear at that moment, to the extent that the friction engagement elements will not be engaged (see Japanese Unexamined Patent Publication No. 10-169764).
In the above-mentioned hydraulic control for discharging the air, however, the hydraulic pressure is supplied even to the friction engagement elements to which the supply of hydraulic pressure should be suspended based on the request in the gear at that moment. Therefore, the hydraulic pressure that is to be supplied to the friction engagement elements becomes deficient, resulting in a possibility of slipping.
It is therefore an object of the present invention to provide a hydraulic control method and an apparatus thereof capable of avoiding such a situation where a hydraulic pressure to be supplied to friction engagement elements that are to be engaged becomes deficient due to an air discharge control.
In order to accomplish the above-mentioned object, according to the present invention, it is estimated whether a hydraulic pressure for friction engagement elements in an engagement state becomes deficient when the hydraulic pressure is supplied to friction engagement elements in a release state so as to discharge air in a hydraulic path, and an air discharge control is executed when it is estimated that the hydraulic pressure does not become deficient.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.